Strategies to improve seed production in Jatropha curcas : a potential seed oil crop for biodiesel.

View/Open

Date

Author

Metadata

Abstract

Interest in planting Jatropha curcas L. for the production of biodiesel is growing
exponentially. The properties of the crop and its oil have persuaded investors to
consider J. curcas oil as a substitute for fossil fuels. However, this plant is still
undomesticated, basic agronomic properties are not thoroughly understood and the
environmental effects on growth have not been investigated. This thesis investigated
different approaches that may contribute to improving the productivity of this plant.
Seed germination and methods of propagation are usually the first consideration in any
plant development programme. The effects of aerosol smoke, smoke water, potassium
nitrate, naphthalene acetic acid and indole-3-butyric acid on germination and seedling
growth of J. curcas were investigated. Seed coat removal accelerated water imbibition
and germination occurred within 48 h. Seeds exposed to aerosol smoke failed to
germinate over the whole study period of three months. There were no significant
differences in total germination between the treatments and the untreated control (intactand
shelled-seed). However, shelled-seeds had a shorter mean germination time. The
seedlings were subsequently sown in trays under shade house conditions and different
seedling growth traits measured after three months. Smoke water, potassium nitrate and
naphthalene acetic acid produced significantly heavier seedlings with longer stems and
roots, wider stems and a higher vigour index compared to the control treatments. Smoke
water, potassium nitrate and naphthalene acetic acid stimulated seedling growth and
vigour of J. curcas. This opens the possibility of applying these treatments to produce
quality seedlings for large scale planting and accelerated plant establishment in
production orchards.
Effective pollination is a prerequisite for many crops to increase seed-set and fruit
production. Experiments were conducted to determine factors that could influence seed
production in this potential biofuel seed crop. Controlled pollination experiments showed
that plants required pollinator visits for seed production and were genetically selfx
compatible. Pollen-supplementation did not lead to increased fruit set, suggesting that
seed production in the study population was not pollen-limited. Both male and female
flowers produced nectar and were highly attractive to honeybees. These insects were
effective pollinators of J. curcas, as shown by experiments in which flowers exposed to
single or multiple visits by honeybees set significantly more fruit than those from which
visits were precluded. Pollinator-mediated self-pollination led to marginally lower levels
of seed production relative to cross-pollination. Progeny from selfed plants had
significantly shorter roots than progeny of outcrossed plants. However, in general, there
was little evidence of inbreeding depression. The present results provide empirical
evidence that honeybees are effective pollinators of J. curcas. Fruit arising from selfpollination
were almost as numerous and as large as those arising from crosspollination,
suggesting that promotion of cross-pollination does not have to be a priority
in orchard management for fruit yield.
Manipulation of pollen development and function is of vital importance for crop
development and improvement. Experiments were conducted to investigate pollen
viability, in vitro pollen germination and in vivo pollen tube growth in J. curcas. Light and
fluorescence microscopy were employed to examine the different developmental
stages. It was possible to determine pollen viability and distinguish between fresh and
dead pollen using 2,3,5-triphenyltetrazolium chloride (TTC). Pollen germination was
significantly higher in an agar-based medium composed of sucrose, boric acid and
calcium nitrate compared with the control treatment (distilled water). Supplementation of
IAA to the different media significantly increased pollen germination and pollen length
compared with the control treatment. Pollen from hermaphrodite flowers had a lower
viability, lower germination rates and shorter pollen tubes, with abnormal shapes,
compared to the pollen from male flowers. Pollen tubes from both self- and crosspollinated
flowers entered the ovary within 8 hours after pollination (HAP). However, at
6 HAP, the pollen tube length and growth rate were significantly higher in crosscompared
to self-pollinated pollen. Our results suggest that TTC is a reliable test for
pollen viability; boric acid, calcium nitrate, sucrose and addition of IAA are essential and
beneficial for pollen germination in this plant. Pollen germination and pollen tube growth
were not inhibited, nor interfered with, as a result of self-pollination treatments. During,
both types of pollination, fertility is maintained as evidenced by ovule penetration by
pollen tubes. This suggests that type of pollination has no influence on the success of
fertilization in J. curcas.
Manual pruning is one of the major management practices in commercial plantations of
J. curcas, resulting in production of more branches and thus increased potential for
more inflorescences leading to a higher seed yield. Experiments were conducted to
determine the response of J. curcas plants to manual pruning under summer and winter
conditions. The results showed that manual pruning under both conditions significantly
increased the number of branches per plant. However, there were no significant
differences in number of branches between winter and summer manual pruning. Winter
pruning, however, had a significantly wider crown diameter compared to the control and
summer pruning. Both treatments produced significantly less fruits/per plant in the
subsequent season compared to the un-pruned control. This study revealed that winter
and summer manual pruning may be suitable practice to promote branching.
Manual pruning, however, is time consuming, labour intensive and expensive. A study
was conducted to determine the potential of different plant growth regulators (PGRs) to
increase the number of lateral branches of J. curcas plants. A single foliar application of
BA (benzyladenine) at 12 mmol l-1 significantly increased branches in both the pot (4)
and field (13.2) trials compared to manual pruning (MP) (1.8 and 5.7 respectively) and
control (no new branches) plants. In the field, treatment with TIBA (2,3,5-triiodobenzoic
acid) (1 mmol l-1) significantly increased the number of branches (15.9) after seven
months from application. Of all the PGRs examined, DK (Dikegulac) (2,3:4,6-di-Oisopropylidene-
2-keto-L-gulonic acid) at 2 mmol l-1 produced the maximum number of
branches (18) in the field seven months after application. Concentrations of 2 and 3
mmol I-1 of MH (Maleic hydrazide) (1,2-dihydro-3,6-pyridazinedione, coline salt)
significantly increased the number of branches, four and seven months after spraying in
both the pot trial in the shade house and field respectively. Under field conditions J.
curcas plants responded better to all the PGRs (DK < TIBA < BA < MH) when treated
once, with insignificant variations of other growth parameters. This study indicates that
a single foliar application of PGRs under field conditions can be an alternative method
to MP for increasing the number of lateral branches of J. curcas plants.
The field chemical pruning experiment was continued to determine the potential
subsequent effects of the different PGRs on seed production. In the subsequent year
following the single foliar application, the parameters of flowering, fruit set, fruit
characteristics, total oil content and free fatty acid (FFA) content were evaluated.
Number of flowers per plant and number of fruits per bunch were significantly affected
by the different treatments. However, there were no variations in the degree of fruit set.
A single foliar application of BA (6-benzylaminopurine) produced more flowers per
plant, more fruits per bunch, heavier and bigger fruits and seeds with more oil
compared to MP (manual pruning). TIBA (2,3,5-Triiodobenzoic acid) produced
significantly more flowers per plant and heavier fruits compared to the control and MP
treatments. However, it produced significantly bigger fruits with more seeds and a
higher oil content than MP. DK (Dikegulac) (2,3:4,6-di-O-isopropylidene-2-keto-Lgulonic
acid) produced more flowers per plant and seeds with high oil content
compared to the control and MP. However, it produced more fruit per bunch and more
seeds per fruit compared to MP. MH (Maleic hydrazide) produced more flowers per
plant, heavier and bigger fruits with numerous, heavier and oil rich seeds compared to
the control and MP. This study indicates that foliar application of PGRs can be used in
J. curcas to increase seed production and improve fruit quality.